Numerous techniques are already known for authenticating papers of value, or the like, with the aid of spot reactions (Dutch patent application No. 6613250) or fluorescence (French Pat. No. 2,289,976; U.S. Pat. No. 4,146,792).
A variety of electronic surveillance of goods or personnel verification systems currently exists. They involve detection of macroscopic magnetic properties (U.S. Pat. No. 5,146,204) or macroscopic resonance associated with LC electrical circuit resonance (U.S. Pat. No. 4,870,391; Japanese Patent No. 4,800,369; Dutch Patent No. 5,068,641; U.S. Pat. No. 5,081,045; Dutch Patent NO. 5,051,727), or bulk mechanical vibration resonance, or nonlinear electrical transponders or high permeability magnetically saturated soft magnetic elements.
A first patent which claimed the application of electron paramagnetic resonance (EPR, also know as electron spin resonance--ESR) to the problems of authenticating or identifying papers of value was U.S. Pat. No. 4,376,264. It teaches the use of substances having EPR characteristics detected by high field EPR in microwave band (from 9 GHz). U.S. Pat. No. 5,149,946 claims the use of ESR in radio-frequency band for solving the same problem. U.S. Pat. No. 5,175,499 claims resolution of the authenticating problem with the help of such magnetic resonance phenomena as nuclear magnetic resonance (NMR), electron spin resonance (ESR), ferromagnetic resonance (FR), ferrimagnetic resonance (FER), antiferrimagnetic resonance (AFER), domain wall resonance (DWR), spin wave resonance (SWR), spin-echoes (SER).
All of the aforementioned patents are based on a variety of magnetic resonance phenomena. These phenomena are associated with nuclear, electron, atomic or molecular magnetic dipole moments acting individually or cooperatively in the presence of external magnetic fields to give nuclear magnetic resonance (NMR), electron spin/paramagnetic resonance (ESR, EPR), ferromagnetic resonance etc. The magnetic resonance is exhibited when the dipole moments precessing in the magnetic fields absorb and re-radiate microwave or radio-frequency electromagnetic radiation at or very close to the precession frequency. The serious disadvantage of applying aforementioned phenomena to the purposes of authenticating and/or identifying objects is necessity of an external static magnetic field for the excitation and detection of a resonance response. This magnetic field can be supplied either as a large field over the entire interrogation volume, or by a small permanent or semi-permanent magnet placed close to the resonant material and carried around with it and the object. For getting resonance responses with high sensitivity and high resolution the said field should be strong (about or more than 0.01 T) and homogeneous. These demands make both the marker/label and instrument design complicated. Further, the presence of a strong external magnetic field within the interrogation volume or near the marker runs the risk of health damage (pacemakers etc.) as well as the risk of wiping out data contained on magnetic media; all this makes it impossible for identifying credit or bank cards.